A structural analysis of the sinoatrial node SAN, the heart's pacemaker, is basic to a better understanding of cardiac arrhythmias, in particular "Sick Sinus Syndromes" (SSS) The quantitative study will utilize light and electron microscope (LM and EM) and histochemical data in a new computer aided stereological analysis of the ferret SAN and ventricular myocardium. For inherent reasons of discrete delineation, small size and extensive innervation, the ferret SAN represents the ideal initial tissue for intensive analysis of the basic SAN organization. A later phase will employ intracellular electrophysiology within the known matrix of SAN organization derived from this study and will involve ultrastructural identification of cells impaled by recording microelectrodes. It will be the subject of a separate grant application. This initial study will quantitatively analyze intra- and intercellular characteristics of SANa1 and perinodal cells and ventricular cells to obtain essential LM and EM morphometric data. It will also provide basic information on the extent and character of the SAN innervation. The aim is to produce a comprehensive picture of the SAN as a data base for subsequent integrated examination with electrophysiologic methodology and a better understanding of the structure of the ventricular heart wall.